Method and apparatus for access in wireless LAN system

ABSTRACT

An embodiment of the present invention provides a method for accessing to a medium by a station (STA) in a wireless LAN system, the method comprising the steps of: receiving a predetermined frame including a time stamp; identifying a restricted access window (RAW) assignment field included in the predetermined frame; and performing access in a slot determined on the basis of a subfield of the RAW assignment field when the STA belongs to a RAW related to the RAW allocation field, wherein whether the STA belongs to the RAW is determined by whether an association identifier (AID) of the STA is included in a AID range and the RAW assignment field includes a subfield indicating whether the AID range is determined by a TIM bitmap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2013/012389, filed on Dec. 30, 2013,which claims the benefit of U.S. Provisional Application No. 61/807,342,filed on Apr. 2, 2013, 61/807,766, filed on Apr. 3, 2013, 61/809,433,filed on Apr. 8, 2013, 61/820,697, filed on May 8, 2013, 61/821,245,filed on May 9, 2013 and 61/845,383, filed on Jul. 12, 2013, thecontents of which are all hereby incorporated by reference herein intheir entirety.

TECHNICAL FIELD

The present disclosure relates to a wireless communication system and,more particularly, to a method and apparatus for access in a WirelessLAN system.

BACKGROUND ART

With recent development of information communication technologies, avariety of wireless communication technologies have been developed. Fromamong such technologies, WLAN is a technology that enables wirelessInternet access at home, in businesses, or in specific service providingareas using a mobile terminal, such as a personal digital assistant(PDA), a laptop computer, or a portable multimedia player (PMP), basedon radio frequency technology.

In order to overcome limited communication speed, which has been pointedout as a weak point of WLAN, technical standards have recentlyintroduced a system capable of increasing the speed and reliability of anetwork while extending coverage of a wireless network. For example,IEEE 802.11n supports high throughput (HT) with a maximum dataprocessing rate of 540 Mbps. In addition, Multiple Input Multiple Output(MIMO) technology, which employs multiple antennas for both atransmitter and a receiver in order to minimize transmission errors andoptimize data rate, has been introduced.

Machine-to-machine (M2M) communication technology has been discussed asa next generation communication technology. A technical standard tosupport M2M communication in the IEEE 802.11 WLAN system is also underdevelopment as IEEE 802.11ah. In M2M communication, a scenario in whicha small amount of data is occasionally communicated at a low speed in anenvironment having a large number of devices may be considered.

Communication in the WLAN system is performed on a medium shared by alldevices. If the number of devices increases as in M2M communication, achannel access mechanism needs to be efficiently improved in order toreduce unnecessary power consumption and interference.

DISCLOSURE Technical Problem

An object of the present invention relates to indication and fieldconfiguration for a situation in which the range of association IDs(AIDs) allocated to a Restricted Access Window (RAW) is identical to theAID range in a Traffic Indication Map (TIM).

Objects of the present invention are not limited to the aforementionedobject, and other objects of the present invention which are notmentioned above will become apparent to those having ordinary skill inthe art upon examination of the following description.

Technical Solution

In a first aspect of the present invention, provided herein is a methodfor performing access to a medium by a station (STA) in a wirelesscommunication system, the method including receiving a predeterminedframe containing a timestamp, checking a Restricted Access Window (RAW)Assignment field contained in the predetermined frame, and performingaccess in a slot determined based on a subfield of the RAW Assignmentfield when the STA belongs to a RAW group related to the RAW Assignmentfield, wherein whether or not the STA belongs to the RAW group isdetermined depending on whether or not an association identifier (AID)of the STA is within an AID range, wherein the RAW Assignment fieldincludes a subfield indicating whether or not the AID range isdetermined by a TIM bitmap.

In a second aspect of the present invention, provided herein is astation (STA) for performing access to a medium in a wirelesscommunication system, the STA including a transceiver module, and aprocessor, wherein the processor is configured to receive apredetermined frame containing a timestamp, check a Restricted AccessWindow (RAW) Assignment field contained in the predetermined frame, andperform access in a slot determined based on a subfield of the RAWAssignment field when the STA belongs to a RAW group related to the RAWAssignment field, wherein whether or not the STA belongs to the RAWgroup is determined depending on whether or not an associationidentifier (AID) of the STA is within an AID range, wherein the RAWAssignment field includes a subfield indicating whether or not the AIDrange is determined by a TIM bitmap.

The first and second aspects of the present invention may include thefollowing details.

When the RAW Assignment field is a first RAW Assignment field of an RPSelement and Same Group Indication is set to 0, the AID range may bedetermined by a RAW Group field.

An Options value contained in a subfield identical to the Same GroupIndication may be set to 0.

STAs included in the AID range may be allowed to perform the access evenif the STA are not paged in the TIM bitmap.

When the RAW Assignment field is a first RAW Assignment field of an RPSelement and Same Group Indication is set to 1, the first RAW Assignmentfield may not include a RAW Group field.

The TIM bitmap may be contained in the predetermined frame.

The predetermined frame may be either a beacon frame or a (short) beaconframe.

When the RAW Assignment field is a second RAW Assignment field or a RAWAssignment field after the second RAW Assignment field in an RPS elementand a Same Group Indication subfield is set to 1, the RAW Assignmentfield may not include a RAW Group field.

The RAW Assignment field may be contained in a RAW Parameter Set (RPS)element.

An RPS element may contain one or more RAW Assignment fields.

Advantageous Effects

According to embodiments of the present invention, the RAW group fieldmay be omitted, and thus the size of the beacon frame may be reduced.

The effects that can be obtained from the present invention are notlimited to the aforementioned effects, and other effects may be clearlyunderstood by those skilled in the art from the descriptions givenbelow.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are intended to provide a furtherunderstanding of the present invention, illustrate various embodimentsof the present invention and together with the descriptions in thisspecification serve to explain the principle of the invention.

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable.

FIG. 3 is a diagram showing yet another exemplary structure of an IEEE802.11 system to which the present invention is applicable.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system.

FIG. 5 illustrates a link setup process in a WLAN system.

FIG. 6 illustrates a backoff process.

FIG. 7 illustrates a hidden node and an exposed node.

FIG. 8 illustrates RTS and CTS.

FIG. 9 illustrates a power management operation.

FIGS. 10 to 12 illustrate operations of a station (STA) having receiveda TIM in detail.

FIG. 13 illustrates a group-based AID.

FIGS. 14 to 16 illustrate a RAW and an RPS element.

FIGS. 17 to 25 illustrate Embodiment 1 of the present invention.

FIGS. 26 to 30 illustrate Embodiment 2 of the present invention.

FIG. 31 is a block diagram illustrating a wireless apparatus accordingto one embodiment of the present invention.

BEST MODE

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings. The detaileddescription, which will be disclosed along with the accompanyingdrawings, is intended to describe exemplary embodiments of the presentinvention and is not intended to describe a unique embodiment throughwhich the present invention can be carried out. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details.

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions or features ofany one embodiment may be included in another embodiment and may bereplaced with corresponding constructions or features of anotherembodiment.

Specific terms used in the following description are provided to aid inunderstanding of the present invention. These specific terms may bereplaced with other terms within the scope and spirit of the presentinvention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems such asthe institute of electrical and electronics engineers (IEEE) 802, 3rdgeneration partnership project (3GPP), 3GPP long term evolution (3GPPLTE), LTE-advanced (LTE-A), and 3GPP2 systems. For steps or parts ofwhich description is omitted to clarify the technical features of thepresent invention, reference may be made to these documents. Further,all terms as set forth herein can be explained by the standarddocuments.

The following technology can be used in various wireless access systemssuch as systems for code division multiple access (CDMA), frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), single carrierfrequency division multiple access (SC-FDMA), etc. CDMA may beimplemented by radio technology such as universal terrestrial radioaccess (UTRA) or CDMA2000. TDMA may be implemented by radio technologysuch as global system for mobile communications (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAmay be implemented by radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, evolved-UTRA (E-UTRA), etc. For clarity,the present disclosure focuses on 3GPP LTE and LTE-A systems. However,the technical features of the present invention are not limited thereto.

Structure of WLAN System

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent station (STA) mobility fora higher layer may be provided by mutual operations of the components. Abasic service set (BSS) may correspond to a basic building block in anIEEE 802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are present and twoSTAs are included in each of the BSSs (i.e. STA1 and STA2 are includedin BSS1 and STA3 and STA4 are included in BSS2). An ellipse indicatingthe BSS in FIG. 1 may be understood as a coverage area in which STAsincluded in a corresponding BSS maintain communication. This area may bereferred to as a basic service area (BSA). If an STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand does not include other components except for the STAs, maycorrespond to a typical example of the IBSS. This configuration ispossible when STAs can directly communicate with each other. Such a typeof LAN may be configured as necessary instead of being prescheduled andis also called an ad-hoc network.

Memberships of an STA in the BSS may be dynamically changed when the STAbecomes an on or off state or the STA enters or leaves a region of theBSS. To become a member of the BSS, the STA may use a synchronizationprocess to join the BSS. To access all services of a BSS infrastructure,the STA should be associated with the BSS. Such association may bedynamically configured and may include use of a distributed systemservice (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a distribution system (DS), a distribution systemmedium (DSM), and an access point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by physical(PHY) performance. In some cases, such restriction of the distance maybe sufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a wireless medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data can be moved betweenthe BSS and the DS through the AP. For example, STA2 and STA3 shown inFIG. 2 have STA functionality and provide a function of causingassociated STAs (STA1 and STA4) to access the DS. Moreover, since allAPs correspond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not necessarilybe identical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may be always received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an extendedservice set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a logical link control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One (or more than one)IBSS or ESS networks may be physically located in the same space as one(or more than one) ESS network. This may correspond to an ESS networkform in the case in which an ad-hoc network operates in a location inwhich an ESS network is present, the case in which IEEE 802.11 networksdifferent organizations physically overlap, or the case in which two ormore different access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as mobile phones, handled directly by users.In FIG. 4, STA1, STA3, and STA4 correspond to the non-AP STAs and STA2and STA5 correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station (MS), a mobile terminal, or a mobile subscriberstation (MSS). The AP is a concept corresponding to a base station (BS),a Node-B, an evolved Node-B (eNB), a base transceiver system (BTS), or afemto BS in other wireless communication fields.

Link Setup Process

FIG. 5 is a diagram for explaining a general link setup process.

In order to allow an STA to establish link setup on a network andtransmit/receive data over the network, the STA should perform processesof network discovery, authentication, association establishment,security setup, etc. The link setup process may also be referred to as asession initiation processor or a session setup process. In addition,discovery, authentication, association, and security setup of the linksetup process may also called an association process.

An exemplary link setup process is described with reference to FIG. 5.

In step S510, an STA may perform a network discovery action. The networkdiscovery action may include an STA scanning action. That is, in orderto access the network, the STA should search for an available network.The STA needs to identify a compatible network before participating in awireless network and the process of identifying the network present in aspecific area is referred to as scanning.

Scanning is categorized into active scanning and passive scanning.

FIG. 5 exemplarily illustrates a network discovery action including anactive scanning process. An STA performing active scanning transmits aprobe request frame in order to determine which AP is present in aperipheral region while moving between channels and waits for a responseto the probe request frame. A responder transmits a probe response framein response to the probe request frame to the STA that has transmittedthe probe request frame. Here, the responder may be an STA that hasfinally transmitted a beacon frame in a BSS of the scanned channel.Since an AP transmits a beacon frame in a BSS, the AP is a responder. Inan IBSS, since STAs of the IBSS sequentially transmit the beacon frame,a responder is not the same. For example, an STA, that has transmittedthe probe request frame at channel #1 and has received the proberesponse frame at channel #1, stores BSS-related information containedin the received probe response frame, and moves to the next channel(e.g. channel #2). In the same manner, the STA may perform scanning(i.e. probe request/response transmission and reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning. An STA that performs passive scanning awaitsreception of a beacon frame while moving from one channel to anotherchannel. The beacon frame is one of management frames in IEEE 802.11.The beacon frame is periodically transmitted to indicate the presence ofa wireless network and allow a scanning STA to search for the wirelessnetwork and thus join the wireless network. In a BSS, an AP isconfigured to periodically transmit the beacon frame and, in an IBSS,STAs in the IBSS are configured to sequentially transmit the beaconframe. Upon receipt of the beacon frame, the scanning STA storesBSS-related information contained in the beacon frame and records beaconframe information on each channel while moving to another channel. Uponreceiving the beacon frame, the STA may store BSS-related informationcontained in the received beacon frame, move to the next channel, andperform scanning on the next channel using the same method.

Active scanning is more advantageous than passive scanning in terms ofdelay and power consumption.

After discovering the network, the STA may perform an authenticationprocess in step S520. The authentication process may be referred to as afirst authentication process in order to clearly distinguish thisprocess from the security setup process of step S540.

The authentication process includes a process in which an STA transmitsan authentication request frame to an AP and the AP transmits anauthentication response frame to the STA in response to theauthentication request frame. The authentication frame used forauthentication request/response corresponds to a management frame.

The authentication frame may include information about an authenticationalgorithm number, an authentication transaction sequence number, a statecode, a challenge text, a robust security network (RSN), a finite cyclicgroup (FCG), etc. The above-mentioned information contained in theauthentication frame may correspond to some parts of information capableof being contained in the authentication request/response frame and maybe replaced with other information or include additional information.

The STA may transmit the authentication request frame to the AP. The APmay determine whether to permit authentication for the corresponding STAbased on the information contained in the received authenticationrequest frame. The AP may provide an authentication processing result tothe STA through the authentication response frame.

After the STA has been successfully authenticated, an associationprocess may be carried out in step S530. The association processincludes a process in which the STA transmits an association requestframe to the AP and the AP transmits an association response frame tothe STA in response to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aservice set identifier (SSID), supported rates, supported channels, anRSN, a mobility domain, supported operating classes, a trafficindication map (TIM) broadcast request, interworking service capability,etc.

For example, the association response frame may include informationassociated with various capabilities, a status code, an association ID(AID), supported rates, an enhanced distributed channel access (EDCA)parameter set, a received channel power indicator (RCPI), a receivedsignal to noise indicator (RSNI), a mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a quality of service (QoS) map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame and may be replaced with other information orinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be performed in step S540. The security setupprocess of step S540 may be referred to as an authentication processbased on robust security network association (RSNA) request/response.The authentication process of step S520 may be referred to as a firstauthentication process and the security setup process of step S540 mayalso be simply referred to as an authentication process.

The security setup process of step S540 may include a private key setupprocess through 4-way handshaking based on, for example, an extensibleauthentication protocol over LAN (EAPOL) frame. In addition, thesecurity setup process may also be performed according to other securityschemes not defined in IEEE 802.11 standards.

WLAN Evolution

To overcome limitations of communication speed in a WLAN, IEEE 802.11nhas recently been established as a communication standard. IEEE 802.11naims to increase network speed and reliability and extend wirelessnetwork coverage. More specifically, IEEE 802.11n supports a highthroughput (HT) of 540 Mbps or more. To minimize transmission errors andoptimize data rate, IEEE 802.11n is based on MIMO using a plurality ofantennas at each of a transmitter and a receiver.

With widespread supply of a WLAN and diversified applications using theWLAN, the necessity of a new WLAN system for supporting a higherprocessing rate than a data processing rate supported by IEEE 802.11nhas recently emerged. A next-generation WLAN system supporting very highthroughput (VHT) is one of IEEE 802.11 WLAN systems which have beenrecently proposed to support a data processing rate of 1 Gbps or more ina MAC service access point (SAP), as the next version (e.g. IEEE802.11ac) of an IEEE 802.11n WLAN system.

To efficiently utilize a radio frequency (RF) channel, thenext-generation WLAN system supports a multiuser (MU)-MIMO transmissionscheme in which a plurality of STAs simultaneously accesses a channel.In accordance with the MU-MIMO transmission scheme, an AP maysimultaneously transmit packets to at least one MIMO-paired STA.

In addition, support of WLAN system operations in whitespace (WS) hasbeen discussed. For example, technology for introducing the WLAN systemin TV WS such as an idle frequency band (e.g. 54 to 698 MHz band) due totransition to digital TVs from analog TVs has been discussed under theIEEE 802.11af standard. However, this is for illustrative purposes only,and the WS may be a licensed band capable of being primarily used onlyby a licensed user. The licensed user is a user who has authority to usethe licensed band and may also be referred to as a licensed device, aprimary user, an incumbent user, etc.

For example, an AP and/or STA operating in WS should provide a functionfor protecting the licensed user. As an example, assuming that thelicensed user such as a microphone has already used a specific WSchannel which is a frequency band divided by regulations so as toinclude a specific bandwidth in the WS band, the AP and/or STA cannotuse the frequency band corresponding to the corresponding WS channel inorder to protect the licensed user. In addition, the AP and/or STAshould stop using the corresponding frequency band under the conditionthat the licensed user uses a frequency band used for transmissionand/or reception of a current frame.

Therefore, the AP and/or STA needs to determine whether a specificfrequency band of a WS band can be used, in other words, whether alicensed user is present in the frequency band. A scheme for determiningwhether a licensed user is present in a specific frequency band isreferred to as spectrum sensing. An energy detection scheme, a signaturedetection scheme, etc. are used as the spectrum sensing mechanism. TheAP and/or STA may determine that the frequency band is being used by alicensed user if the intensity of a received signal exceeds apredetermined value or if a DTV preamble is detected.

Machine-to-machine (M2M) communication technology has been discussed asnext generation communication technology. Technical standard forsupporting M2M communication has been developed as IEEE 802.11ah in anIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines or may also be called machine typecommunication (MTC) or machine-to-machine communication. In this case,the machine refers to an entity that does not require directmanipulation or intervention of a user. For example, not only a meter orvending machine including a radio communication module but also a userequipment (UE) such as a smartphone capable of performing communicationby automatically accessing a network without usermanipulation/intervention may be machines. M2M communication may includedevice-to-device (D2D) communication and communication between a deviceand an application server. As exemplary communication between a deviceand an application server, communication between a vending machine andan application server, communication between a point of sale (POS)device and an application server, and communication between an electricmeter, a gas meter, or a water meter and an application server. M2Mcommunication-based applications may include security, transportation,healthcare, etc. In the case of considering the above-mentionedapplication examples, M2M communication has to support occasionaltransmission/reception of a small amount of data at low speed under anenvironment including a large number of devices.

More specifically, M2M communication should support a large number ofSTAs. Although a currently defined WLAN system assumes that one AP isassociated with a maximum of 2007 STAs, methods for supporting othercases in which more STAs (e.g. about 6000 STAs) than 2007 STAs areassociated with one AP have been discussed in M2M communication. Inaddition, it is expected that many applications forsupporting/requesting a low transfer rate are present in M2Mcommunication. In order to smoothly support these requirements, an STAin the WLAN system may recognize the presence or absence of data to betransmitted thereto based on a TIM element and methods for reducing thebitmap size of the TIM have been discussed. In addition, it is expectedthat much traffic having a very long transmission/reception interval ispresent in M2M communication. For example, a very small amount of datasuch as electric/gas/water metering needs to be transmitted and receivedat long intervals (e.g. every month). Accordingly, although the numberof STAs associated with one AP increases in the WLAN system, methods forefficiently supporting the case in which there are a very small numberof STAs each including a data frame to be received from the AP duringone beacon period has been discussed.

As described above, WLAN technology is rapidly developing and not onlythe above-mentioned exemplary technologies but also other technologiesincluding direct link setup, improvement of media streaming throughput,support of high-speed and/or large-scale initial session setup, andsupport of extended bandwidth and operating frequency are beingdeveloped.

Medium Access Mechanism

In a WLAN system based on IEEE 802.11, a basic access mechanism ofmedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordination function (DCF) of the IEEE802.11 MAC and basically adopts a “listen before talk” access mechanism.In this type of access mechanism, an AP and/or an STA may sense awireless channel or a medium during a predetermined time duration (e.g.DCF interframe space (DIFS) before starting transmission. As a result ofsensing, if it is determined that the medium is in an idle status, theAP and/or the STA starts frame transmission using the medium. Meanwhile,if it is sensed that the medium is in an occupied state, the AP and/orthe STA does not start its transmission and may attempt to perform frametransmission after setting and waiting for a delay duration (e.g. arandom backoff period) for medium access. Since it is expected thatmultiple STAs attempt to perform frame transmission after waiting fordifferent time durations by applying the random backoff period,collision can be minimized.

An IEEE 802.11 MAC protocol provides a hybrid coordination function(HCF) based on the DCF and a point coordination function (PCF). The PCFrefers to a scheme of performing periodic polling by using apolling-based synchronous access method so that all reception APs and/orSTAs can receive a data frame. The HCF includes enhanced distributedchannel access (EDCA) and HCF controlled channel access (HCCA). EDCA isa contention based access scheme used by a provider to provide a dataframe to a plurality of users. HCCA uses a contention-free based channelaccess scheme employing a polling mechanism. The HCF includes a mediumaccess mechanism for improving QoS of a WLAN and QoS data may betransmitted in both a contention period (CP) and a contention-freeperiod (CFP).

FIG. 6 is a diagram for explaining a backoff process.

Operations based on a random backoff period will now be described withreference to FIG. 6. If a medium of an occupy or busy state transitionsto an idle state, several STAs may attempt to transmit data (or frames).As a method for minimizing collision, each STA may select a randombackoff count, wait for a slot time corresponding to the selectedbackoff count, and then attempt to start data or frame transmission. Therandom backoff count may be a pseudo-random integer and may be set toone of 0 to CW values. In this case, CW is a contention window parametervalue. Although CWmin is given as an initial value of the CW parameter,the initial value may be doubled in case of transmission failure (e.g.in the case in which ACK for the transmission frame is not received). Ifthe CW parameter value reaches CWmax, the STAs may attempt to performdata transmission while CWmax is maintained until data transmission issuccessful. If data has been successfully transmitted, the CW parametervalue is reset to CWmin. Desirably, CW, CWmin, and CWmax are set to2^(n)−1 (where n=0, 1, 2, . . . ).

If the random backoff process is started, the STA continuously monitorsthe medium while counting down the backoff slot in response to thedetermined backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle status, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at STA3, STA3 may confirm that the medium is in the idlestate during a DIFS and directly start frame transmission. In themeantime, the remaining STAs monitor whether the medium is in the busystate and wait for a predetermined time. During the predetermined time,data to be transmitted may occur in each of STA1, STA2, and STA5. If itis monitored that the medium is in the idle state, each STA waits forthe DIFS time and then may perform countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupationof STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS and restarts backoffcounting. That is, after counting down the remaining backoff timecorresponding to the residual backoff time, each of STA1 and STA5 maystart frame transmission. Since the residual backoff time of STA5 isshorter than that of STA1, STA5 starts frame transmission. Meanwhile,data to be transmitted may occur even in STA4 while STA2 occupies themedium. In this case, if the medium is in the idle state, STA4 may waitfor the DIFS time, perform countdown in response to the random backoffcount value selected thereby, and then start frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, collision may occur between STA4 and STA5. Then, each of STA4 andSTA5 does not receive ACK, resulting in occurrence of data transmissionfailure. In this case, each of STA4 and STA5 may increase the CW valueby two times, select a random backoff count value, and then performcountdown. Meanwhile, STA1 waits for a predetermined time while themedium is in the occupied state due to transmission of STA4 and STA5. Ifthe medium is in the idle state, STA1 may wait for the DIFS time andthen start frame transmission after lapse of the residual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or an STA directly sensesa medium but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems such as a hidden nodeproblem encountered in medium access. For virtual carrier sensing, MACof the WLAN system may use a network allocation vector (NAV). The NAV isa value used to indicate a time remaining until an AP and/or an STAwhich is currently using the medium or has authority to use the mediumenters an available state to another AP and/or STA. Accordingly, a valueset to the NAV corresponds to a reserved time in which the medium willbe used by an AP and/or STA configured to transmit a correspondingframe. An STA receiving the NAV value is not allowed to perform mediumaccess during the corresponding reserved time. For example, NAV may beset according to the value of a ‘duration’ field of a MAC header of aframe.

A robust collision detection mechanism has been proposed to reduce theprobability of collision. This will be described with reference to FIGS.7 and 8. Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of description.

FIG. 7 is a diagram for explaining a hidden node and an exposed node.

FIG. 7(a) exemplarily shows a hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted.Specifically, STA C may determine that a medium is in an idle state whenperforming carrier sensing before transmitting data to STA B, althoughSTA A is transmitting information to STA B. This is because transmissionof STA A (i.e. occupation of the medium) may not be detected at thelocation of STA C. In this case, STA B simultaneously receivesinformation of STA A and information of STA C, resulting in occurrenceof collision. Here, STA A may be considered a hidden node of STA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 7(b), in asituation in which STA B transmits data to STA A, STA C has informationto be transmitted to STA D. If STA C performs carrier sensing, it isdetermined that a medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,since the medium-occupied state is sensed, STA C should wait for apredetermined time until the medium is in the idle state. However, sinceSTA A is actually located out of the transmission range of STA C,transmission from STA C may not collide with transmission from STA Bfrom the viewpoint of STA A, so that STA C unnecessarily enters astandby state until STA B stops transmission. Here, STA C is referred toas an exposed node of STA B.

FIG. 8 is a diagram for explaining request to send (RTS) and clear tosend (CTS).

To efficiently utilize a collision avoidance mechanism under theabove-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS and CTS. RTS/CTS between two STAs may beoverheard by peripheral STA(s), so that the peripheral STA(s) mayconsider whether information is transmitted between the two STAs. Forexample, if an STA to be used for data transmission transmits an RTSframe to an STA receiving data, the STA receiving data may informperipheral STAs that itself will receive data by transmitting a CTSframe to the peripheral STAs.

FIG. 8(a) exemplarily shows a method for solving problems of a hiddennode. In FIG. 8(a), it is assumed that both STA A and STA C are ready totransmit data to STA B. If STA A transmits RTS to STA B, STA B transmitsCTS to each of STA A and STA C located in the vicinity of the STA B. Asa result, STA C waits for a predetermined time until STA A and STA Bstop data transmission, thereby avoiding collision.

FIG. 8(b) exemplarily shows a method for solving problems of an exposednode. STA C performs overhearing of RTS/CTS transmission between STA Aand STA B, so that STA C may determine that no collision will occuralthough STA C transmits data to another STA (e.g. STA D). That is, STAB transmits RTS to all peripheral STAs and only STA A having data to beactually transmitted may transmit CTS. STA C receives only the RTS anddoes not receive the CTS of STA A, so that it can be recognized that STAA is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system needs to perform channel sensingbefore an STA performs data transmission/reception. The operation ofalways sensing the channel causes persistent power consumption of theSTA. Power consumption in a reception state is not greatly differentfrom that in a transmission state. Continuous maintenance of thereception state may cause large load to a power-limited STA (i.e. an STAoperated by a battery). Therefore, if an STA maintains a receptionstandby mode so as to persistently sense a channel, power isinefficiently consumed without special advantages in terms of WLANthroughput. In order to solve the above-mentioned problem, the WLANsystem supports a power management (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a powersave (PS) mode. The STA basically operates in the active mode. The STAoperating in the active mode maintains an awake state. In the awakestate, the STA may perform a normal operation such as frametransmission/reception or channel scanning. On the other hand, the STAoperating in the PS mode is configured to switch between a sleep stateand an awake state. In the sleep state, the STA operates with minimumpower and performs neither frame transmission/reception nor channelscanning.

Since power consumption is reduced in proportion to a specific time inwhich the STA stays in the sleep state, an operation time of the STA isincreased. However, it is impossible to transmit or receive a frame inthe sleep state so that the STA cannot always operate for a long periodof time. If there is a frame to be transmitted to an AP, the STAoperating in the sleep state is switched to the awake state totransmit/receive the frame. On the other hand, if the AP has a frame tobe transmitted to the STA, the sleep-state STA is unable to receive theframe and cannot recognize the presence of a frame to be received.Accordingly, the STA may need to switch to the awake state according toa specific period in order to recognize the presence or absence of aframe to be transmitted thereto (or in order to receive the frame if theAP has the frame to be transmitted thereto).

FIG. 9 is a diagram for explaining a PM operation.

Referring to FIG. 9, an AP 210 transmits a beacon frame to STAs presentin a BSS at intervals of a predetermined time period (S211, S212, S213,S214, S215, and S216). The beacon frame includes a TIM informationelement. The TIM information element includes buffered traffic regardingSTAs associated with the AP 210 and includes information indicating thata frame is to be transmitted. The TIM information element includes a TIMfor indicating a unicast frame and a delivery traffic indication map(DTIM) for indicating a multicast or broadcast frame.

The AP 210 may transmit a DTIM once whenever the beacon frame istransmitted three times. Each of STA1 220 and STA2 222 operate in a PSmode. Each of STA1 220 and STA2 222 is switched from a sleep state to anawake state every wakeup interval of a predetermined period such thatSTA1 220 and STA2 222 may be configured to receive the TIM informationelement transmitted by the AP 210. Each STA may calculate a switchingstart time at which each STA may start switching to the awake statebased on its own local clock. In FIG. 9, it is assumed that a clock ofthe STA is identical to a clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state when the AP 210 first transmits the beacon frame (S211).STA1 220 may receive the beacon frame and obtain the TIM informationelement. If the obtained TIM element indicates the presence of a frameto be transmitted to STA1 220, STA1 220 may transmit a power save-Poll(PS-Poll) frame, which requests the AP 210 to transmit the frame, to theAP 210 (S221 a). The AP 210 may transmit the frame to STA1 220 inresponse to the PS-Poll frame (S231). STA1 220 which has received theframe is re-switched to the sleep state and operates in the sleep state.

When the AP 210 secondly transmits the beacon frame, since a busy mediumstate in which the medium is accessed by another device is obtained, theAP 210 may not transmit the beacon frame at an accurate beacon intervaland may transmit the beacon frame at a delayed time (S212). In thiscase, although STA1 220 is switched to the awake state in response tothe beacon interval, STA1 does not receive the delay-transmitted beaconframe so that it re-enters the sleep state (S222).

When the AP 210 thirdly transmits the beacon frame, the correspondingbeacon frame may include a TIM element configured as a DTIM. However,since the busy medium state is given, the AP 210 transmits the beaconframe at a delayed time (S213). STA1 220 is switched to the awake statein response to the beacon interval and may obtain a DTIM through thebeacon frame transmitted by the AP 210. It is assumed that the DTIMobtained by STA1 220 does not have a frame to be transmitted to STA1 220and there is a frame for another STA. In this case, STA1 220 may confirmthe absence of a frame to be received in the STA1 220 and re-enters thesleep state so that the STA1 220 may operate in the sleep state. Aftertransmitting the beacon frame, the AP 210 transmits the frame to thecorresponding STA (S232).

The AP 210 fourthly transmits the beacon frame (S214). However, since itwas impossible for STA1 220 to obtain information regarding the presenceof buffered traffic associated therewith through previous doublereception of a TIM element, STA1 220 may adjust the wakeup interval forreceiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by the AP 210, the wakeupinterval value of the STA1 220 may be adjusted. In this example, STA1220, which has been switched to receive a TIM element every beaconinterval, may be configured to be switched to another operation state inwhich STA1 220 awakes from the sleep state once every three beaconintervals. Therefore, when the AP 210 transmits a fourth beacon frame(S214) and transmits a fifth beacon frame (S215), STA1 220 maintains thesleep state such that it cannot obtain the corresponding TIM element.

When the AP 210 sixthly transmits the beacon frame (S216), STA1 220 isswitched to the awake state and operates in the awake state, so that theSTA1 220 may obtain the TIM element contained in the beacon frame(S224). The TIM element is a DTIM indicating the presence of a broadcastframe. Accordingly, STA1 220 does not transmit the PS-Poll frame to theAP 210 and may receive the broadcast frame transmitted by the AP 210(S234). In the meantime, the wakeup interval configured for STA2 230 maybe longer than the wakeup interval of STA1 220. Accordingly, STA2 230may enter the awake state at a specific time (S215) where the AP 210fifthly transmits the beacon frame and receives the TIM element (S241).STA2 230 may recognize the presence of a frame to be transmitted theretothrough the TIM element and transmit the PS-Poll frame to the AP 210 torequest frame transmission (S241 a). The AP 210 may transmit the frameto STA2 230 in response to the PS-Poll frame (S233).

In order to manage a PS mode shown in FIG. 9, the TIM element mayinclude either a TIM indicating the presence or absence of a frame to betransmitted to the STA or include a DTIM indicating the presence orabsence of a broadcast/multicast frame. The DTIM may be implementedthrough field setting of the TIM element.

FIGS. 10 to 12 are diagrams for explaining detailed operations of an STAthat has received a TIM.

Referring to FIG. 10, an STA is switched from a sleep state to an awakestate so as to receive a beacon frame including a TIM from an AP. TheSTA may recognize the presence of buffered traffic to be transmittedthereto by interpreting the received TIM element. After contending withother STAs to access a medium for PS-Poll frame transmission, the STAmay transmit the PS-Poll frame for requesting data frame transmission tothe AP. Upon receiving the PS-Poll frame transmitted by the STA, the APmay transmit the frame to the STA. The STA may receive a data frame andthen transmit an ACK frame to the AP in response to the received dataframe. Thereafter, the STA may re-enter the sleep state.

As illustrated in FIG. 10, the AP may operate according to an immediateresponse scheme in which the AP receives the PS-Poll frame from the STAand transmits the data frame after a predetermined time (e.g. a shortinterframe space (SIFS)). Meanwhile, if the AP does not prepare a dataframe to be transmitted to the STA during the SIFS time after receivingthe PS-Poll frame, the AP may operate according to a deferred responsescheme and this will be described with reference to FIG. 11.

The STA operations of FIG. 11 in which an STA is switched from a sleepstate to an awake state, receives a TIM from an AP, and transmits aPS-Poll frame to the AP through contention are identical to those ofFIG. 10. Even upon receiving the PS-Poll frame, if the AP does notprepare a data frame during an SIFS time, the AP may transmit an ACKframe to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of contention. TheSTA may transmit the ACK frame indicating that the data frame hassuccessfully been received to the AP and transition to the sleep state.

FIG. 12 illustrates an exemplary case in which an AP transmits a DTIM.STAs may be switched from the sleep state to the awake state so as toreceive a beacon frame including a DTIM element from the AP. The STAsmay recognize that a multicast/broadcast frame will be transmittedthrough the received DTIM. After transmission of the beacon frameincluding the DTIM, the AP may directly transmit data (i.e. themulticast/broadcast frame) without transmitting/receiving a PS-Pollframe. While the STAs continuously maintains the awake state afterreception of the beacon frame including the DTIM, the STAs may receivedata and then switch to the sleep state after completion of datareception.

TIM Structure

In the operation and management method of the PS mode based on the TIM(or DTIM) protocol described with reference to FIGS. 9 to 12, STAs maydetermine whether a data frame to be transmitted for the STAs throughSTA identification information contained in a TIM element. The STAidentification information may be information associated with an AID tobe allocated when an STA is associated with an AP.

The AID is used as a unique ID of each STA within one BSS. For example,the AID for use in the current WLAN system may be allocated as one of 1to 2007. In the currently defined WLAN system, 14 bits for the AID maybe allocated to a frame transmitted by an AP and/or an STA. Although theAID value may be assigned up to 16383, the values of 2008 to 16383 areset to reserved values.

A TIM element according to legacy definition is inappropriate to applyan M2M application through which many STAs (for example, more than 2007STAs) are associated with one AP. If a conventional TIM structure isextended without any change, since the TIM bitmap size excessivelyincreases, it is impossible to support the extended TIM structure usinga legacy frame format and the extended TIM structure is inappropriatefor M2M communication in which application of a low transfer rate isconsidered. In addition, it is expected that there are a very smallnumber of STAs each having a reception data frame during one beaconperiod. Therefore, according to exemplary application of theabove-mentioned M2M communication, since it is expected that most bitsare set to zero (0) although the TIM bitmap size is increased,technology capable of efficiently compressing a bitmap is needed.

In legacy bitmap compression technology, successive values of 0 areomitted from a front part of a bitmap and the omitted result may bedefined as an offset (or start point) value. However, although STAs eachincluding a buffered frame is small in number, if there is a highdifference between AID values of respective STAs, compression efficiencyis not high. For example, assuming that only a frame to be transmittedto two STAs having AID values of 10 and 2000 is buffered, the length ofa compressed bitmap is set to 1990 but the remaining parts other thanboth end parts are assigned zero. If fewer STAs are associated with oneAP, inefficiency of bitmap compression does not cause serious problems.However, if the number of STAs associated with one AP increases, suchinefficiency may deteriorate overall system performance.

In order to solve the above-mentioned problems, AIDs are divided into aplurality of groups such that data can be more efficiently transmitted.A designated group ID (GID) is allocated to each group. AIDs allocatedon a group basis will be described with reference to FIG. 13.

FIG. 13(a) is a diagram illustrating an exemplary group-based AID. InFIG. 13(a), a few bits located at the front part of an AID bitmap may beused to indicate a GID. For example, it is possible to designate fourGIDs using the first two bits of an AID bitmap. If a total length of theAID bitmap is N bits, the first two bits (B1 and B2) may represent a GIDof the corresponding AID.

FIG. 13(a) is a diagram illustrating another exemplary group-based AID.In FIG. 13(b), a GID may be allocated according to the position of theAID. In this case, AIDs having the same GID may be represented by offsetand length values. For example, if GID 1 is denoted by offset A andlength B, this means that AIDs of A to A+B−1 on a bitmap have GID 1. Forexample, FIG. 13(b) assumes that AIDs of 1 to N4 are divided into fourgroups. In this case, AIDs contained in GID 1 are denoted by 1 to N1 andthe AIDs contained in this group may be represented by offset 1 andlength N1. Next, AIDs contained in GID 2 may be represented by offsetN1+1 and length N2−N1+1, AIDs contained in GID 3 may be represented byoffset N2+1 and length N3−N2+1, and AIDs contained in GID 4 may berepresented by offset N3+1 and length N4−N3+1.

If the aforementioned group-based AIDs are introduced, channel accessmay be allowed in a different time interval according to GIDs, so thatthe problem caused by the insufficient number of TIM elements withrespect to a large number of STAs can be solved and at the same timedata can be efficiently transmitted/received. For example, during aspecific time interval, channel access is allowed only for STA(s)corresponding to a specific group and channel access to the remainingSTA(s) may be restricted. A predetermined time interval in which accessto only specific STA(s) is allowed may also be referred to as arestricted access window (RAW).

Channel access based on GID will now be described with reference to FIG.13(c). FIG. 13(c) exemplarily illustrates a channel access mechanismaccording to a beacon interval when AIDs are divided into three groups.A first beacon interval (or a first RAW) is a specific interval in whichchannel access to STAs corresponding to AIDs contained in GID 1 isallowed and channel access of STAs contained in other GIDs isdisallowed. To implement this, a TIM element used only for AIDscorresponding to GID 1 is contained in a first beacon. A TIM elementused only for AIDs corresponding to GID 2 is contained in a secondbeacon frame. Accordingly, only channel access to STAs corresponding tothe AIDs contained in GID 2 is allowed during a second beacon interval(or a second RAW). A TIM element used only for AIDs having GID 3 iscontained in a third beacon frame, so that channel access to STAscorresponding to the AIDs contained in GID 3 is allowed during a thirdbeacon interval (or a third RAW). A TIM element used only for AIDshaving GID 1 is contained in a fourth beacon frame, so that channelaccess to STAs corresponding to the AIDs contained in GID 1 is allowedduring a fourth beacon interval (or a fourth RAW). Thereafter, onlychannel access to STAs belonging to a specific group indicated by a TIMcontained in a corresponding beacon frame may be allowed in each ofbeacon intervals subsequent to the fifth beacon interval (or in each ofRAWs subsequent to the fifth RAW).

Although FIG. 13(c) exemplarily shows that the order of allowed GIDs iscyclical or periodic according to the beacon interval, the scope of thepresent invention is not limited thereto. That is, only AID(s) containedin specific GID(s) may be contained in a TIM element, so that channelaccess only to STA(s) corresponding to the specific AID(s) is allowedduring a specific time interval (e.g. a specific RAW) and channel accessto the remaining STA(s) is disallowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, a total AIDspace is divided into a plurality of blocks and channel access to STA(s)(i.e. STA(s) of a specific group) corresponding to a specific blockhaving any one of values other than ‘0’ may be allowed. Therefore, sincea large-sized TIM is divided into small-sized blocks/groups, an STA caneasily maintain TIM information and blocks/groups may be easily managedaccording to class, QoS or usage of the STA. Although FIG. 13exemplarily shows a 2-level layer, a hierarchical TIM structurecomprised of two or more levels may be configured. For example, a totalAID space may be divided into a plurality of page groups, each pagegroup may be divided into a plurality of blocks, and each block may bedivided into a plurality of sub-blocks. In this case, according to theextended version of FIG. 13(a), first N1 bits of an AID bitmap mayrepresent a page ID (i.e. PID), the next N2 bits may represent a blockID, the next N3 bits may represent a sub-block ID, and the remainingbits may represent the position of STA bits contained in a sub-block.

In the embodiments of the present invention described below, variousschemes for dividing STAs (or AIDs allocated to the STAs respectively)into predetermined hierarchical group units and managing the same may beused, but the group-based AID allocation schemes are not limited tothese embodiments.

Restricted Access Window (RAW)

Collision occurring between STAs that perform access simultaneously mayreduce medium utilization. Accordingly, a RAW may be used as a method todistribute channel access from (group-based) STAs. An AP may assign amedium access interval called RAW between beacon intervals. RAW-relatedinformation (a Restricted Access Window Parameter Set (RPS) element) maybe transmitted in a (short) beacon frame. In addition to the RAW, the APmay further assign one or more different RAWs related to other RAWparameters for another group between the beacon intervals.

FIG. 14 shows an exemplary RAW. Referring to FIG. 14, STAs of a specificgroup corresponding to a RAW may perform access in the RAW (morespecifically, in one of the slots of the RAW). Herein, the specificgroup may be indicated by, for example, an RAW Group field, which willbe described later. In other words, an STA may recognize whether the AIDthereof corresponds to a specific group (RAW group) by determiningwhether or not the AID is within an AID range indicated by, for example,the RAW Group field. For example, if the AID of the STA is greater thanor equal to the lowest AID(N1) allocated to the RAW and less than orequal to the highest AID(N1) allocated to the RAW, the STA may beconsidered as belonging to a RAW group indicated by the RAW Group field.Herein, N1 may be determined by a concatenation of a Page Index subfieldand an RAW Start AID subfield, and N2 may be determined by aconcatenation of the Page Index subfield and an RAW End AID subfield.The subfields may be included in a RAW Group subfield in the RPSelement.

If the STA corresponds to the RAW group illustrated in FIG. 14 (and ispaged), the STA may perform access by transmitting a PS-Poll frame basedon the DCF and EDCA in the slot allocated thereto. Herein, the allocatedslot may be a slot allocated by the AP among the slots included in theRAW. The slot may be allocated in a manner as shown in FIG. 15. In FIGS.15(a) and 15(b), a slot is basically determined byi_(slot)=(x+N_(offset))mod N_(RAW), wherein x is the AID of the STA,i_(slot) is the slot index allocated to the STA, N_(offset) denotes twoleast significant bytes (LSBs) of an FCS field of the (short) beaconframe, and N_(RAW) is the number of time slots included in the RAW,which may be determined by a RAW Slot Definition subfield in the RPSelement. FIG. 15(a) illustrates allocation of slots to AIDs performedregardless of whether the AID is set to 1 in the TIM bitmap, and FIG.15(b) illustrates allocation of slots to only AIDs set to 1 in the TIMbitmap.

Restricted Access Window Parameter Set (RPS) Element

The RPS element includes a parameter set necessary for the RAW describedabove. This information field includes RAW Assignment fields for Groups1 to N. FIG. 16 shows an RPS element. Specifically, FIG. 16(a) showfields constituting the RPS element, FIG. 16(b) shows subfieldsconstituting the RAW N Assignment field, FIG. 16(c) shows configurationof a RAW Group subfield among the subfields of the RAW N Assignmentfield, and FIG. 16(d) shows configuration of an Options subfield amongthe subfields of the RAW N Assignment field.

Referring to FIG. 16(a), the RPS element may include an Element IDfield, a Length field, and a RAW N Assignment field.

Referring to FIG. 16(b), the RAW N Assignment field may include a PRAWIndication subfield, a Same Group Indication subfield, a RAW Groupsubfield, a RAW Start Time subfield, a RAW Duration subfield, an Optionssubfield, and a RAW Slot Definition subfield.

The PRAW Indication subfield indicates whether the current RAWAssignment field is a normal RAW or a PRAW. The Same Group Indicationsubfield indicates whether a RAW group related to the current RAWAssignment field is the same as the RAW group defined in the previousRAW Assignment field. If the Same Group Indication subfield is set to 1,this indicates that the RAW group of the current RAW Assignment field isthe same as the RAW group defined in the previous RAW Assignment field.In this case, the current RAW Assignment field does not include the RAWGroup field. The RAW Group subfield indicates the AID range of the STAsof the group related to the current RAW Assignment field. As shown inFIG. 16(c), the RAW Group field may include Page Index, RAW Start AIDand RAW End AID subfields. Since description of how the range of AID isdetermined by these subfields has been given above in relation to theRAW, it will not be given below.

The RAW Start Time subfield indicates time from the end time of beacontransmission to the start time of the RAW in units of TU. The RAWDuration subfield indicates the duration, in TU, of restricted mediumaccess which is allocated to the RAW. The Options subfield includes anAccess Restricted to Paged STAs Only subfield, which indicates whetheronly paged STAs are allowed to perform access in the RAW. The RAW SlotDefinition subfield may include a Slot Duration subfield, a SlotAssignments subfield, and a Cross Slot Boundary subfield. For details ofinformation which is included in the RPS element but is not describedabove and information/fields which are not specifically described above,refer to IEEE P802.11ah/D0.1.

As described above, the RAW is used as a method for allocating durationsfor channel access to STAs belonging to a specific group. Whether or notan STA belongs to the specific group is basically determined by the AIDrange indicated by the RAW Group field. However, if the TIM is containedin the beacon frame and slots of the RAW are allocated to the STAsindicated by the TIM, the RPS element does not necessarily include theRAW Group field for the RAW. In other words, if the AID range determinedby the TIM bitmap is the same as the AID range indicated/determined bythe RAW Group field, the RAW Group field may be omitted. This means thatthe bits of the RPS element may be reduced by 24×n bits and also meansthat the size of the beacon frame may be reduced. Hereinafter, relevantembodiments of the present invention will be described in detail.

Embodiment 1

As described above, the Same Group Indication subfield included in theRPS element indicates whether a RAW group related to the current RAWAssignment field is the same as the RAW group defined in the previousRAW Assignment field. Accordingly, if the current RAW Assignment fieldis the first RAW Assignment field of the RPS element, there is no RAWAssignment field preceding the current RAW Assignment field. Therefore,if the current RAW Assignment field is the first RAW Assignment field ofthe RPS element, the Same Group Indication subfield is always set to 0.If the current RAW Assignment field is the first RAW Assignment field ofthe RPS element, and the Same Group Indication subfield is set to 1,this may indicate that the AID range of the RAW group related to thecurrent RAW Assignment field is the same as the information on the STAscontained in the TIM bitmap. That is, if the current RAW Assignmentfield is the first RAW Assignment field of the RPS element, and the SameGroup Indication subfield is set to 1, the AID range to be determined bythe RAW Group field may be set to be determined by the TIM bitmap. Inthis case, the range of AIDs corresponding to the RAW group related tothe current RAW Assignment field is indicated by the TIM bitmapcontained in the (short) beacon frame, and therefore the RPS element,more specifically, the RAW Assignment field, need not include a RAWGroup subfield.

With the configuration as above, an STA performs access as follows. TheSTA receives a predetermined frame including a timestamptimestamp, i.e.,a (short) beacon frame, and checks the RAW Assignment field included inthe predetermined frame. If the STA belongs to a RAW group related tothe RAW Assignment field, it may perform access in a time slotdetermined based on the subfields of the RAW Assignment field. Herein,whether or not the STA belongs to the RAW group is determined dependingon whether or not the AID of the STA is within the AID range. If the RAWAssignment field is the second RAW Assignment field or a RAW Assignmentfield after the second RAW Assignment field in the RPS element, therange of AIDs may be determined by the RAW Group field. If the RAWAssignment field is the first RAW Assignment field in the RPS element,and the Same Group Indication is set to 1, the range of AIDs may bedetermined by the TIM bitmap. The access of the STA described aboverelates to a situation in which the first bit of the Options subfieldincluded in the same subfield containing the Same Group Indication(i.e., Bit 0, the Access Restricted to Paged STAs Only subfield) is setto 0. Regarding operation of the STA described above, the STA performsaccess if the AID of the STA is within the AID range. This is becauseaccess is allowed once the first bit of the Options subfield is set to 0even when the STA in the AID range is not paged in the TIM bitmap. Ifthe first bit (Bit 0) of the Options subfield is set to 1, channelaccess is allowed only when the STA is paged in the TIM. Additionally,when the RAW Assignment field is the second RAW Assignment field or aRAW Assignment field after the second RAW Assignment field in the RPSelement, the range of AIDs may be determined by the RAW Group field ifthe Same Group Indication is set to 0 and determined by the previous RAWAssignment field if the Same Group Indication is set to 1. In this case,if the Same Group Indication is set to 1 in the first RAW Assignmentfield, the Same Group Indication of the second or further RAW Assignmentfield set to 1 indicates that the AID range for the second or furtherRAW is also the same as the AID range indicated by the TIM bitmap.

In summary, if the Same Group Indication of the first RAW Assignmentfield is set to 1, this indicates that the RAW group of the first RAWAssignment field is determined by the TIM bitmap of a (short) beaconframe. This in turn indicates that the TIM bitmap is included in the(short) beacon frame, the Page Index is set to the page index of the TIMbitmap, the RAW Start AID subfield is set to the lowest AID in the TIMbitmap, and the RAW End AID subfield is set to the highest AID in theTIM bitmap. When two or more TIM bitmaps are included in a beacon, theSame Group Indication bit in the first RAW Assignment field ispreferably set to 1 only if Bit 0 of the Options subfield is set to 1. Aspecific method may be determined when the AP is implemented. In thiscase, a RAW Start AID and a relevant Page Index are set to the lowestAID in the first TIM bitmap and the page index of the TIM bitmap, whilea RAW End AID and a relevant Page Index are set to the highest AID inthe TIM bitmap and the page index of the TIM bitmap.

FIG. 17 illustrates the RAW Assignment field described above. As shownin FIG. 17, if Same Group Indication 1702 of RAW 1 Assignment field 1701is set to 1, this means that a RAW group AID range related to RAW 1 isdetermined by a TIM bitmap 1703 contained in the beacon frame. In FIG.17, the AID range in the TIM bitmap corresponds to AIDs 1 to 8, and thusthe AID range of the RAW group related to the RAW 1 Assignment fieldalso corresponds to AIDs 1 to 8. If information of the RAW Groupsubfield is determined based on the information on the STAs indicated bythe TIM bitmap, the field may be configured as the RAW Group subfield1704 shown in the figure. In the example of FIG. 17, the Optionssubfield (a subfield indicting whether or not access is restricted topaged STAs only, which may be an Access Restricted to Paged STAs Onlysubfield) is set to 1, namely only paged STAs 1, 2, 4 and 5 in the TIMbitmap are assigned the slots of the RAW. Herein, the relationshipbetween the STAs and slot assignment may be different from therelationship illustrated in the figure. Hereinafter, various variants ofthe examples described above will be described with reference to FIGS.18 to 26. Descriptions of FIGS. 18 to 26 are based on the comprehensivedescription of Embodiment 1 given above. If not specifically mentionedotherwise, the details of FIG. 17 are applied to the variants.

FIG. 18 illustrates an example with a TIM bitmap for Page 1 (AID rangeof AID 1 to AID 8) and a TIM bitmap for Page 2 (AID range of AID 33 toAID 40). When the Same Group Indication subfield is set to 1, the AIDrange of a RAW group related to the RAW 1 Assignment field may be theAID ranges corresponding to the TIM bitmaps for Page 1 and Page 2, i.e.,AIDs 1 to 8 and 33 to 40. When the RAW Group field is used, two RAWGroup subfields as shown in the figure may be used. If the Option is setto 0, RAW 1 may be assigned to STAs 1 to 8 and 33 to 40, which isdifferent from the illustrated example.

FIG. 19 illustrates an example with a TIM bitmap for Page 1 (AID rangeof AID 1 to AID 8) and a TIM bitmap for Page 2 (AID range of AID 33 toAID 40) as in the example of FIG. 18. Contrary to the example of FIG.18, the AID range corresponds to the first AID indicated by the TIMbitmap to the AID of the last paged STA indicated by the TIM bitmap.That is, the AID range covers AIDs 1 to 5 and AIDs 33 to 37.

In the example of FIG. 20, the first block of the TIM bitmap for Page 1includes STA information for AIDs 1 to 8, and another block of the TIMbitmap for Page 1 includes STA information for AIDs 33 to 40. In FIG.20, when the Same Group Indication subfield is set to 1, the AID rangeof a RAW group related to the RAW 1 Assignment field may correspond toAIDs 1 to 8 and 33 to 40. When the RAW Group field is used, two RAWGroup subfields as shown in the figure may be used. If the value of theAccess Restricted to Page STAs Only bit corresponding to Bit 0 in theOptions subfield is 0, RAW 1 may be assigned to STAs 1 to 8 and 33 to40, in contrast with the illustrated example.

FIG. 21 illustrates an example similar to that of FIG. 20. In thisexample, the AID range covers the first AID of each block in a TIMbitmap to the AID of the last paged STA of each block in the TIM bitmap.That is, in FIG. 21, the AID range corresponds to AIDs 1 to 5 and AIDs33 to 37.

The example of FIG. 22 is different from that of FIG. 20 only in thatthe AID range covers the first AID of the first block in the TIM bitmapto the last AID of the last block in the TIM bitmap. That is, all STAsbetween the two blocks may be designated as STAs belonging to the RAWgroup.

In the example of FIG. 23, AIDs between blocks are included in the AIDrange as in the example of FIG. 22. However, the example of FIG. 23 isdifferent from that of FIG. 22 in that the start AID is the AID of thefirst paged STA of the first block and the last AID is the last pagedAID of the last block. That is, in FIG. 23, the AID range corresponds toAIDs 2 to 37.

FIG. 24 also illustrates an example in which AIDs between blocks areincluded in the AID range as in the examples above. However, the exampleof FIG. 24 is different from the examples above in that the start AID isthe AID of the first STA of the first block, and the last AID is thelast paged AID of the last block.

FIG. 25 illustrates another example in which the AID range correspondsto the first AID to a paged AID in the TIM bitmap. In other words, theAID range corresponds to AIDs 1 to 5. FIG. 26 illustrates anotherexample having TIM bitmaps for two pages and an AID range correspondingto AIDs 1 to 5 and AIDs 33 to 37.

Embodiment 2

Unlike Embodiment 1 utilizing the Same Group Indication subfield,Embodiment 2 employs a new field (a Same TIM Indication subfield)indicating whether or not the RAW group related to the RAW Assignmentfield is the same as the group information indicated by the TIM IE. TheSame TIM Indication subfield may indicate whether or not the RAW hasbeen assigned the same information as the information on all STAs (groupand AID information on STAs). FIG. 27 illustrates a RAW Assignment fieldincluding the Same TIM Indication subfield.

If the Same TIM Indication subfield is set to 1, this means that groupinformation, which is the same as the information on all the STAsindicated by the TIM, is given. In this case, the AP may omit the RAWgroup subfield from the RPS element. If the Same TIM Indication subfieldis set to 0, this means that the group information in the TIM isunrelated to the current RAW. Accordingly, a separate RAW group subfieldis needed. FIG. 28 shows examples of a RAW Assignment field for theaforementioned two cases.

FIGS. 29 and 30 illustrate application of the RAW Assignment fieldaccording to Embodiment 2. Referring to FIG. 29, since the Same TIMIndication subfield is set to 1, AIDs 1 to 8, which is the range of AIDsindicated by the TIM bitmap, may correspond to the RAW group of the RAWAssignment field. In particular, FIG. 29 illustrates assignment of theRAW to only paged STAs in the TIM with the Options set to 1.

FIG. 30 illustrates application of the RAW Assignment field with a TIMbitmap for Page 1 and a TIM bitmap for Page 2. Since the Same TIMIndication subfields for the TIM bitmap are both set to 1, the AID rangeof a RAW group related to each RAW Assignment field is indicated by theTIM for each page.

Table 1 given below shows bits necessary for a RAW Assignment fieldprior to Embodiment 2, and Table 2 shows bits which are needed for a RAWAssignment field when Embodiment 2 is applied.

TABLE 1 Feature Value (bits) IE type 8 IE length 8 PRAW Indication (0) 1Same Group Indication 1 Page ID 2 RAW Start AID 11 RAW End AID 11 RAWStart Time 8 RAW Duration 8 Access restriction 1 Frame Type Restriction1 Group/RA frame indication 1 RAW Slot definition 12 Channel 8 AP PM 1Reserved 6 Total: 88

TABLE 2 Feature Value (bits) IE type 8 IE length 8 PRAW Indication (0) 1Same TIM Indication 1 Same Group Indication 1 Page ID 2 RAW Start AID 11RAW End AID 11 RAW Start Time 8 RAW Duration 8 Access restriction 1Frame Type Restriction 1 Group/RA frame indication 1 RAW Slot definition12 Channel 8 AP PM 1 Reserved 5 Total: 88

According to Tables 1 and 2, if one TIM IE is transmitted (i.e., a TIMIE is transmitted for one page), and two RAWs are assigned, theconventional method needs IE Type & Length (16 bits)+2×RAW N Assignment(72 bits)=20 bytes (160 bits). On the other hand, in Embodiment 2, IEType & Length (16 bits)+2×RAW N Assignment (48 bits)=14 bytes (112 bits)are needed, and thus the number of bits may be reduced by 6 bytes(Gain=20% overhead reduction).

If two TIM IEs are transmitted (i.e., TIM IEs are transmitted for twopage), and two RAWs are assigned, a PS-Poll RAW and a data RAW areassigned to each page, the conventional method needs IE Type & Length(16 bits)+4×RAW N Assignment (72 bits)=38 bytes (304 bits). On the otherhand, in Embodiment 2, IE Type & Length (16 bits)+2×RAW N Assignment (48bits)=14 bytes (112 bits) are needed, and thus the number of bits may bereduced by 24 bytes (Gain=63% overhead reduction).

Details of various embodiments of the present invention described abovemay be independently employed or a combination of two or moreembodiments may be implemented.

Configuration of Apparatus According to Embodiment of the PresentInvention

FIG. 31 is a block diagram illustrating wireless apparatuses accordingto one embodiment of the present invention.

An AP 10 may include a processor 11, a memory 12, and a transceiver 13.An STA 20 may include a processor 21, a memory 22, and a transceiver 23.The transceivers 13 and 23 may transmit/receive wireless signals andimplement, for example, a physical layer according to an IEEE 802system. The processors 11 and 21 may be connected to the transceivers 13and 23 to implement a physical layer and/or MAC layer according to anIEEE 802 system. The processors 11 and 21 may be configured to performoperations according to the various embodiments of the present inventiondescribed above. In addition, modules to implement operations of the APand STA according to the various embodiments of the present inventiondescribed above may be stored in the memories 12 and 22 and executed bythe processors 11 and 21. The memories 12 and 22 may be contained in orinstalled outside the processors 11 and 21 and connected to theprocessors 11 and 21 via well-known means.

Configuration of the AP and the STA may be implemented such that thedetails of the various embodiments of the present invention describedabove are independently applied or a combination of two or moreembodiments is applied. For clarity, redundant description is omitted.

Embodiments of the present invention may be implemented by various meanssuch as, for example, hardware, firmware, software, or combinationsthereof.

When embodied as hardware, methods according to embodiments of thepresent invention may be implemented by one or more ASICs (applicationspecific integrated circuits), DSPs (digital signal processors), DSPDs(digital signal processing devices), PLDs (programmable logic devices),FPGAs (field programmable gate arrays), processors, controllers,microcontrollers, microprocessors, and the like.

When embodied as firmware or software, methods according to embodimentsof the present invention may be implemented in the form of a module, aprocedure, a function, or the like which performs the functions oroperations described above. Software code may be stored in the memoryunit and executed by the processor. The memory unit may be disposedinside or outside the processor to transceive data with the processorvia various well-known means.

Preferred embodiments of the present invention have been described indetail above to allow those skilled in the art to implement and practicethe present invention. Although the preferred embodiments of the presentinvention have been described above, those skilled in the art willappreciate that various modifications and variations can be made in thepresent invention without departing from the spirit and scope of theinvention set forth in the claims below. Thus, the present invention isnot intended to be limited to the embodiments described herein, but isintended to include the widest range of embodiments corresponding to theprinciples and novel features disclosed herein.

INDUSTRIAL APPLICABILITY

Various embodiments of the present invention have been described throughexamples applied to an IEEE 802.11 system, but they may also be appliedto other wireless access systems in the same manner.

What is claimed is:
 1. A method for performing access to a medium by astation, STA, in a wireless communication system, the method comprising:checking a Restricted Access Window, RAW, Assignment field among aplurality of RAW Assignment fields included in a RAW Parameter Set, RPS,element of a beacon frame; and performing the access in a slotdetermined based on a first subfield of the RAW Assignment field whenthe STA belongs to a RAW group related to the RAW Assignment field amongthe plurality of RAW Assignment fields, wherein whether or not the STAbelongs to the RAW group is determined depending on whether or not anassociation identifier, AID, of the STA is within an AID range, whereineach of the plurality of RAW Assignment fields comprises a secondsubfield indicating whether an AID range defined in each of the RAWAssignment fields is a same AID range that is defined in a previous RAWAssignment field of each of the RAW Assignment fields, wherein, when theRAW Assignment field is a first RAW Assignment field, having no previousRAW Assignment field, of the RPS element, the second subfield includedin the first RAW Assignment field indicates whether or not the AID rangedefined in the first RAW Assignment field is determined by a trafficindication map, TIM, bitmap, and wherein, when the RAW Assignment field,having no previous RAW Assignment field, is the first RAW Assignmentfield of the RPS element and the second subfield included in the firstRAW Assignment field indicating whether or not the AID range isdetermined by the TIM bitmap is set to a first value, the AID rangedefined in the first RAW Assignment field is determined by the TIMbitmap.
 2. The method according to claim 1, wherein determining the AIDrange by the TIM bitmap comprises setting the AID range to be identicalto an AID range of all TIM bitmaps in the beacon frame.
 3. The methodaccording to claim 1, wherein the second subfield indicating whether ornot the AID range is determined by the TIM bitmap is a Same GroupIndication subfield.
 4. The method according to claim 1, wherein, whenthe second subfield of the first RAW Assignment field indicates that theAID range is determined by the TIM bitmap, the first RAW Assignmentfield does not comprise a RAW Group subfield indicating STA AIDs thatare allowed access within a slot determined based on the first subfieldof the first RAW Assignment field.
 5. The method according to claim 1,wherein, when the RAW Assignment field is the first RAW Assignmentfield, having no previous RAW Assignment field, of an RPS element andthe second subfield included in the first RAW Assignment fieldindicating whether or not the AID range is determined by the TIM bitmapis set to a second value, the AID range defined in the first RAWAssignment field is determined by a RAW Group subfield indicating STAAIDs that are allowed access within a slot determined based on the firstsubfield of the first RAW Assignment field.
 6. The method according toclaim 1, wherein an Options value contained in a subfield identical tothe second subfield indicating whether or not the AID range isdetermined by the TIM bitmap is set to a second value.
 7. The methodaccording to claim 6, wherein STAs included in the AID range are allowedto perform the access even if the STAs are not paged in the TIM bitmap.8. The method according to claim 1, wherein the TIM bitmap is containedin the beacon frame.
 9. The method according to claim 1, wherein, whenthe RAW Assignment field is a second RAW Assignment field having aprevious RAW Assignment field in an RPS element and the second subfieldincluded in the second RAW Assignment field is set to the first value,the RAW Assignment field does not comprise a RAW Group field indicatingSTA AIDs that are allowed access within a slot determined based on thefirst subfield of the second RAW Assignment field.
 10. A station, STA,for performing access to a medium in a wireless communication system,the STA comprising: a transceiver module; and a processor, wherein theprocessor is configured to: check a Restricted Access Window, RAW,Assignment field among a plurality of RAW Assignment fields included ina RAW Parameter Set, RPS, element of a beacon frame; and perform theaccess in a slot determined based on a first subfield of the RAWAssignment field when the STA belongs to a RAW group related to the RAWAssignment field among the plurality of RAW Assignment fields, whereinwhether or not the STA belongs to the RAW group is determined dependingon whether or not an association identifier, AID, of the STA is withinan AID range, wherein each of the plurality of RAW Assignment fieldscomprises a second subfield indicating whether an AID range defined ineach of the RAW Assignment fields is a same AID range that is defined ina previous RAW Assignment field of each of the RAW Assignment fields,wherein, when the RAW Assignment field is a first RAW Assignment field,having no previous RAW Assignment field, of the RPS element, the secondsubfield included in the first RAW Assignment field indicates whether ornot the AID range defined in the first RAW Assignment field isdetermined by a traffic indication map, TIM, bitmap, and wherein, whenthe RAW Assignment field is the first RAW Assignment field, having noprevious RAW Assignment field, of the RPS element and the secondsubfield included in the first RAW Assignment field indicating whetheror not the AID range is determined by the TIM bitmap is set to a firstvalue, the AID range defined in the first RAW Assignment field isdetermined by the TIM bitmap.